1. Field of the Invention
The present invention relates generally to printed circuit devices; and, more particularly, to a flexible printed circuit device having a relatively rigid electronic component mounting portion and a relatively pliable connection wiring portion.
2. Description of the Prior Art
In portable electronic equipment such as cameras, there is a need to incorporate complex electronic circuits in the relatively small space that is available inside such equipment. As such equipment has become more compact and multi-functional, printed circuit devices incorporating flexible substrates which can be freely bent have come to be used. However, due to its pliability, a flexible substrate tends not to remain flat when an electronic component is mounted on one side thereof; and, as a result, it is often not possible to properly mount electronic components on both sides of the substrate.
A rigid substrate, on the other hand, is capable of having electronic components properly mounted on both sides because it will remain flat even after an electronic component is mounted on one side. However, due to its lack of pliability, it cannot be bent freely and, thus, often cannot be fit into a small space.
FIG. 6 illustrates a known printed circuit device which comprises a multi-layer structure in an area where an electronic component 82 is mounted, and a single-layer structure which functions as a connection wiring portion 85 for connection of the device with another printed circuit device. The multi-layer structure is relatively rigid and includes bases 71, 73 and 76; circuit pattern layers 72, 74, 75 and 77; and cover films 78 and 79. Openings 86 provide for electrical interconnection between the circuit pattern layers as illustrated in the Fig.
The single-layer structure includes bases 71 and 73 and circuit pattern layer 72, and is formed using a pliable material.
However, even though the multi-layer structure is relatively rigid, if the single-layer connection wiring portion 85 is bent, bending stresses are applied to and tend to deform the multi-layer structure where electronic component 82 is mounted; and these bending stresses can cause the welding 83 of electronic component 82 to break and come off as shown in FIG. 6. Therefore, in a construction as shown in FIG. 6, it is necessary to provide a sufficient distance between the connection wiring portion and the portion where electronic component 82 is mounted in order to dissipate such bending stresses.
When pliability is needed in a multi-layer printed circuit device on which electronic components can be mounted close together, the following substrate constructions have been proposed: (1) substrates constructed such that rigid substrate portions and flexible substrate portions are connected via electrode terminals having spaces which allow precision, and (2) substrates having substrate layers which are rigid in the vicinity where electronic components are mounted but which are flexible in other areas. Proposed construction (2) includes, for example, substrates which are made pliable through a construction in which a rigid glass epoxy material and a pliable polyimide material are attached to each other to form the substrate, and in which electronic components are mounted on the rigid portion of the substrate formed of the glass epoxy material; and the bending area, which requires pliability, is formed of the polyimide material only by removing the glass epoxy material.
In substrates formed by connecting rigid substrate portions and flexible substrate portions via connecting terminals, as described in proposed construction (1) above, the total area of the substrate increases because a special area is required for the connecting terminals. Moreover, additional connecting work is required, thereby increasing the overall cost of the printed circuit device.
In substrates made by gluing together a rigid glass epoxy material and a pliable polyimide material as in proposed construction (2) above, on the other hand, the adhesion of the two substrates must be ensured, taking into consideration the different coefficients of thermal expansion of the two materials which gives rise to questionable reliability.
In addition, there is a process for removing the highly rigid substrate material from the bending portion which requires pliability. During this process, if residue from the removal process is left on a high-density, rigid substrate, unexpected problems such as defective operation of the circuits may occur. This necessitates a high level of care during processing and increases the cost of the substrate.